1. Field of the Invention
The present invention generally relates to a heat sink for a control device in an automobile air conditioning system, and more particularly, to the positioning of a control device on the evaporator of an air conditioning refrigerant circuit so that heat is effectively transferred from the control device without impeding the flow of air through a duct which connects the evaporator to the automobile passenger compartment.
2. Description of the Prior Art
In general, an automobile air conditioning system includes a refrigerant circuit and an electric circuit comprising electric devices for operating various elements of the refrigerant circuit. For example, a condenser fan motor operates in conjunction with a condenser of the refrigerant circuit, and a control apparatus which includes a control device, such as a power transistor, which is included with the electric circuit, which controls the operational condition of the condenser fan motor. Electric control devices likewise control various other electric devices which operate various refrigerant elements of the refrigerant circuit. However, heat generated in the control device during operation may damage the control device thereby detrimentally affecting control of the air conditioning system. Therefore, the control device must be cooled during operation of the air conditioning system to prevent thermal damage.
FIG. 1 shows a portion of a conventional automobile air conditioning system. Referring to FIG. 1, power transistor 100 is the control device of an electric control circuit. Power transistor 100 is mounted on a heat sink which includes a plurality of radiating plates 110 firmly secured to a top surface thereof, and is firmly attached to an inner surface of cover member 120 by a pair of screws 121. Cooling unit 50 comprises an evaporator (not shown) and casing 500 which is air tight and surrounds the evaporator while allowing air to pass through a heat exchanging portion of the evaporator. Casing 500 comprises two openings (only opening 501 is shown) opposing each other. Opening 501 of cooling unit 50 is opened to the outside, and the other opening is air tight and connected to one end (not shown) of duct 130. Duct 130 comprises opening 131 formed at a side surface thereof adjacent cooling unit 50. Opening 131 is adapted to accommodate power transistor 100 and its associated heat sink in duct 130. Cover member 131 forms an airtight seal around opening 131 by using a plurality of screws 132.
In operation, the air outside cooling unit 50 is conducted into cooling unit 50 through opening 501 by operation of an evaporator fan (not shown) as depicted by arrow "A". The air in cooling unit 50 is cooled while passing through a heat exchanging portion of the evaporator. The cooled air subsequently flows into duct 130 through the opening of casing 500, opposite the air inlet, to be conducted into the passenger compartment of an automobile as depicted by arrow "B". In this arrangement, a part of the cooled air in duct 130 is heat exchanged with plates 110 so as to prevent an excessive ries in the temperature of power transistor 100. Thus, thermal damage to power transistor 100 (the control device) is prevented and control of the air conditioning system is not detrimentally affected.
However, in certain conventional automobile air conditioning systems, where electric devices are used which consume large amounts of electric power, the amount of heat generated in power transistor 100 is increased. Therefore, radiating plates 110 on the heat sink associated with power transistor 100 must have increased dimensions in order to prevent an excessive rise in the temperature of power transistor 100. The result is that air in duct 130 is not able to flow smoothly due to interference with both power transistor 100 and plates 110. Therefore, the evaporator fan wastefully consumes electric power in order to compensate for the interference with the flow of air in duct 130 caused by power transistor 100 and plates 110.
FIG. 2 shows a block diagram of a refrigerant circuit of an automobile air conditioning system. Referring to FIG. 2, refrigerant circuit 200 includes compressor 210, condenser 220, receiver-drier 230, thermostatic expansion valve 240 and evaporator 250 serially connected through pipe members 270. Cooling device 260 is disposed between evaporator 250 and compressor 210 through pipe members 270. Cooling device 260 has a power transistor incorporated therein, as the control device, of an electric control circuit for the automobile air conditioning system. The throttling condition of expansion valve 240 is varied in response to the temperature at an outlet of evaporator 250, located upstream with respect to cooling device 260, in order to maintain super heat at the outlet of evaporator 250 at a proper temperature. In this construction, the power transistor is cooled by virtue of heat exchange between the suction refrigerant gas flowing through cooling device 260 in order to prevent the excessive rise in temperature of the power transistor. In this way, thermal damage to the power transistor (the control device) is prevented.
However, in this automobile air conditioning system, further hermetic joints are required in the refrigerant circuit in order to dispose cooling device 260 between evaporator 250 and compressor 210. Therefore, the process of assembling the refrigerant circuit becomes more complicated.